[ovs-discuss] Double free in recent kernels after memleak fix

[Joel Fernandes]

Hi,
Adding more of us working on RCU as well. Johan from another team at
Google discovered a likely issue in openswitch, details below:

On Fri, Aug 7, 2020 at 11:32 AM Johan Knöös <jknoos at google.com> wrote:
>
> On Tue, Aug 4, 2020 at 8:52 AM Gregory Rose <gvrose8192 at gmail.com> wrote:
> >
> >
> >
> > On 8/3/2020 12:01 PM, Johan Knöös via discuss wrote:
> > > Hi Open vSwitch contributors,
> > >
> > > We have found openvswitch is causing double-freeing of memory. The
> > > issue was not present in kernel version 5.5.17 but is present in
> > > 5.6.14 and newer kernels.
> > >
> > > After reverting the RCU commits below for debugging, enabling
> > > slub_debug, lockdep, and KASAN, we see the warnings at the end of this
> > > email in the kernel log (the last one shows the double-free). When I
> > > revert 50b0e61b32ee890a75b4377d5fbe770a86d6a4c1 ("net: openvswitch:
> > > fix possible memleak on destroy flow-table"), the symptoms disappear.
> > > While I have a reliable way to reproduce the issue, I unfortunately
> > > don't yet have a process that's amenable to sharing. Please take a
> > > look.
> > >
> > > 189a6883dcf7 rcu: Remove kfree_call_rcu_nobatch()
> > > 77a40f97030b rcu: Remove kfree_rcu() special casing and lazy-callback handling
> > > e99637becb2e rcu: Add support for debug_objects debugging for kfree_rcu()
> > > 0392bebebf26 rcu: Add multiple in-flight batches of kfree_rcu() work
> > > 569d767087ef rcu: Make kfree_rcu() use a non-atomic ->monitor_todo
> > > a35d16905efc rcu: Add basic support for kfree_rcu() batching
> > >

Note that these reverts were only for testing the same code, because
he was testing 2 different kernel versions. One of them did not have
this set. So I asked him to revert. There's no known bug in the
reverted code itself. But somehow these patches do make it harder for
him to reproduce the issue.

> > > Thanks,
> > > Johan Knöös
> >
> > Let's add the author of the patch you reverted and the Linux netdev
> > mailing list.
> >
> > - Greg
>
> I found we also sometimes get warnings from
> https://elixir.bootlin.com/linux/v5.5.17/source/kernel/rcu/tree.c#L2239
> under similar conditions even on kernel 5.5.17, which I believe may be
> related. However, it's much rarer and I don't have a reliable way of
> reproducing it. Perhaps 50b0e61b32ee890a75b4377d5fbe770a86d6a4c1 only
> increases the frequency of a pre-existing bug.

This is interesting, because I saw kbuild warn me recently [1] about
it as well. Though, I was actually intentionally messing with the
segcblist. I plan to debug it next week, but the warning itself is
unlikely to be caused by my patch IMHO (since it is slightly
orthogonal to what I changed).

[1] https://lore.kernel.org/lkml/20200720005334.GC19262@shao2-debian/

But then again, I have not heard reports of this warning firing. Paul,
has this come to your radar recently?

Thanks,

 - Joel